function remSize(){
     var deviceWidth=document.documentElement.clientWidth||window.innerWidth
     var deviceHeight=document.documentElement.clientHeight||window.innerHeight
     if(deviceWidth>750){
        deviceWidth=750
     }
     if(deviceWidth<=320){
        deviceWidth=320
     }
     //750px==>1rem=100px
     document.documentElement.style.fontSize=(deviceWidth/7.5)+"px"
     document.querySelector("body").style.fontSize=0.3+"rem"
     document.querySelector("body").style.height=deviceHeight
}
remSize()
window.onresize=()=>{
remSize()
}